1. Field of the Invention
The present invention relates to a marine propulsion device.
2. Description of the Related Art
To reduce fuel consumption, marine propulsion devices such as outboard motors have been demanded to achieve high compression within the combustion chambers of an engine. However, a load acting on a crankshaft is increased due to such high compression.
In Japan Laid-open Patent Application Publication No. JP-A-S62-165014, axial support portions (journals) of a crankshaft, which are required to be durable, are made of a material with a durability higher than that of the other portions. The axial support portions are connected to the other portions by welding. JP-A-S62-165014 discloses that the fatigue strength of the crankshaft is enhanced due to the above-described structure. JP-A-S62-165014 also discloses that the manufacturing cost can be kept low compared to a case in which the crankshaft is entirely made of a highly durable material.
However, in such a crankshaft as described in JP-S62-165014, a plurality of components are required to be welded to each other after being manufactured. In this case, the number of manufacturing steps is increased. Therefore, the manufacturing cost cannot be necessarily kept low.
On the other hand, the strength of a crankshaft can be enhanced by enlarging its size and shape without changing its material. However, enlarging the size and shape of a crankshaft results in an increase in the weight of the crankshaft. This goes against the original objective, that is, to reduce fuel consumption. Further, in an outboard motor, for instance, an engine is disposed within an engine cover. Therefore, the engine size is constrained by the size of the engine cover. In some cases, a plurality of outboard motors are mounted in alignment on a vessel body. Under this condition, it is difficult to enlarge the engine cover and still reliably produce adequate steering ranges for adjacent outboard motors. Thus, it is also not easy to enlarge the engine itself. In view of this, it is also difficult to enlarge the size and shape of the crankshaft.
Moreover, the strength of a crankshaft can be also enhanced by executing a high strengthening treatment on the entire crankshaft. However, a soft nitriding treatment is conventionally performed on normal crankshafts of marine propulsion devices. The soft nitriding treatment is a treatment intended to mainly enhance corrosion resistance and abrasion resistance. The soft nitriding treatment also includes an advantageous effect of enhancing the strength. Therefore, a sufficient strength can be achieved for crankshafts of current marine propulsion devices only with the soft nitriding treatment.
However, it is insufficient to perform only the soft nitriding treatment to obtain a strength required for a crankshaft in which high compression is achieved within the combustion chambers. In view of this, it is possible to perform a high strengthening treatment such as induction hardening with respect to the entire crankshaft in addition to a gas soft nitriding treatment. In this case, however, the crankshaft is greatly affected and thermally expanded by the induction hardening treatment. Therefore, after the thermal treatment, a polishing treatment is required to be performed on those portions requiring accurate dimensions. Thus, the number of manufacturing steps is inevitably increased as a whole.
Further, a functional portion is disposed on an end of a crankshaft of a marine propulsion device in order to transmit power to another functional component different from the crankshaft. For example, the functional portion is a spline or a gear to couple the crankshaft to a drive shaft. Alternatively, the functional portion is, for instance, a gear to drive a cam belt or a timing belt. When an induction hardening treatment is performed on the entire crankshaft including the functional portion, thermal expansion is caused due to the induction hardening treatment. Thermal expansion produces a drawback in that the accuracy in the axial center of the functional portion is degraded and the function of the functional portion is deteriorated.
Furthermore, when a polishing treatment is performed after an induction hardening treatment in order to enhance the accuracy in the axial center, a nitride layer is inevitably eliminated through the polishing treatment because the nitride layer has quite a small thickness. In the crankshaft, the functional portion (e.g., a spline, a gear, etc.) is not necessarily positioned in an area filled with a lubricating oil. Especially in an engine of a marine propulsion device, chances are that such a functional portion is exposed to an atmosphere including seawater. When the nitride layer is eliminated through the polishing treatment, a drawback is produced in that the functional portion loses a corrosive-resistant function. Therefore, in terms of corrosion resistance, deterioration in the function of the functional portion is also inevitably caused by executing the induction hardening treatment on the entire crankshaft.
It should be noted that not only in executing an induction hardening treatment but also in executing a high strengthening treatment such as a rolling treatment, a polishing treatment is required because deformation results from the induction hardening treatment or the high strengthening treatment. Therefore, a drawback is produced similarly to the above.